1. Field of the Invention
The present invention relates to a substrate holding technique adapted, for example, to exposure apparatuses used for fabricating devices having micro-patterns such as LSIs (large-scale integrations). In particular, the present invention relates to a substrate-holding technique used in, for example, a liquid-immersion exposure apparatus for projecting a pattern of an original onto a photosensitive substrate via a liquid layer.
2. Description of the Related Art
A typical liquid-immersion exposure apparatus is provided with a light source and an optical projection unit to achieve higher resolution or higher focal depth. Japanese Patent Laid-Open Nos. 6-124873 and 6-168866 disclose examples of such a liquid-immersion exposure apparatus.
A conventional technique related to the present invention will now be described with reference to FIGS. 9 to 13.
FIG. 9 illustrates a typical liquid-immersion exposure apparatus, which includes an illumination unit 1; a reticle stage 2; a step-and-repeat lens 3; a main body 4; and a wafer stage 5. The illumination unit 1 receives a light beam from a light source, not shown in the drawing, and emits the light beam towards a reticle, which is not shown in the drawing, provided in the reticle stage 2. The reticle includes an exposure pattern of an original. The reticle stage 2 photoscans the reticle against a wafer 12 shown in FIG. 10, i.e. a photosensitive substrate, at a predetermined reduction rate. The step-and-repeat lens 3 projects the original pattern included in the reticle towards the wafer 12 at a reduced rate. The main body 4 supports the reticle stage 2, the step-and-repeat lens 3, and the wafer stage 5. The wafer stage 5 shifts the wafer 12 to an exposure position in a step-by-step manner, and scans the wafer 12 in synchronization with the scanning of the reticle.
Referring to FIG. 10, the wafer stage 5 includes a stage base 5S, a wafer chuck 5C, and a slider 5B. The wafer chuck 5C is disposed on the slider 5B. The wafer chuck 5C holds the wafer 12 in place and also contains immersion liquid. The wafer chuck 5C is movable while holding the wafer 12 in place, and moreover, the wafer chuck 5C can be detached from the slider 5B.
The exposure apparatus further includes an alignment scope 6; a focus scope 7; an x-axis mirror bar 8; and a y-axis mirror bar 9.
The alignment scope 6 is a microscope which measures an alignment mark on the wafer 12 and an alignment reference mark on the wafer stage 5 so as to determine the alignment position of the wafer 12 with respect to a reference point and the alignment position between the reticle and the wafer 12. The focus scope 7 determines the surface structure of the wafer 12 and performs a focus measurement in an optical-axis direction (positional measurement on the surface of the wafer 12 with respect to the optical-axis direction).
The x-axis mirror bar 8 functions as a target for determining the position of the slider 5B in the x-axis direction via a laser interferometer. On the other hand, the y-axis mirror bar 9 functions as a target for determining the position of the slider 5B in the y-axis direction.
The upper surface of the slider 5B is provided with a stage reference marker 10 and a light-intensity sensor 11. The stage reference marker 10 is provided with a target used for stage alignment. The light-intensity sensor 11 calibrates the intensity of light before an exposure operation so as to compensate the light intensity. The pattern in the reticle is projected and transferred to the wafer 12 via the step-and-repeat lens 3. Thus, the wafer 12, i.e. a single-crystal silicon substrate, has a resist pattern disposed thereon.
In FIG. 9, the wafer chuck 5C and the wafer 12 on the slider 5B are illustrated in cross-section so as to provide an easier understanding of the relationship among the step-and-repeat lens 3, a light beam from the focus scope 7, and the wafer 12.
The exposure apparatus further includes a wafer-shifting robot 13 for supplying and retrieving the wafer 12; an immersion-liquid tank 14 for storing the immersion liquid; and a liquid supplying/retrieving unit 15 for supplying the immersion liquid to the wafer chuck 5C from the immersion-liquid tank 14 or retrieving the immersion liquid from the wafer chuck 5C back to the immersion-liquid tank 14.
FIGS. 11A to 11H illustrate the liquid-immersion exposure operation performed on the wafer 12.
Referring to FIG. 11A, an alignment operation is performed in the wafer stage 5 so that an alignment measurement and a focus measurement can be performed on the wafer 12 disposed on the wafer stage 5. In this state, the immersion liquid is not present between the wafer 12 and the step-and-repeat lens 3. If the immersion liquid is present, the measurements become difficult since the difference in the index of refraction between the resist and the immersion liquid is small. For this reason, the alignment measurement and the focus measurement are performed without any of the immersion liquid being supplied.
Referring to FIG. 11B, after the alignment operation is completed, the wafer 12 on the wafer stage 5 is shifted to a position directly below the liquid supplying/retrieving unit 15. After this shifting process, the immersion-liquid tank 14 supplies immersion liquid having a refraction of index of more than 1 to the liquid supplying/retrieving unit 15. The liquid supplying/retrieving unit 15 then supplies droplets of the immersion liquid to the wafer chuck 5C until the liquid layer on the surface of the wafer 12 reaches a predetermined thickness. Accordingly, liquid having a refraction of index greater than the air fills a space provided between the step-and-repeat lens 3 and the wafer 12 such that the numerical aperture of the step-and-repeat lens 3 is enlarged. This contributes to higher resolution.
Referring to FIG. 11C, the wafer 12 is shifted to an exposure position in a state where the immersion liquid is disposed on the wafer 12. Subsequently, referring to FIG. 11D, a step-and-repeat exposure process or a step-and-scan exposure process is performed. Referring to FIG. 11E, after the exposure process is completed, the slider 5B is shifted to a retrieving position of the wafer-shifting robot 13 such that the wafer 12 is retrieved. Referring to FIG. 11F, after the retrieving process, the retrieved wafer 12 is ejected from the exposure apparatus in order to perform the subsequent step.
Referring to FIG. 11G, a subsequent wafer 12 is transferred to the wafer chuck 5C on the wafer stage 5 via the wafer-shifting robot 13. Referring to FIG. 11H, the subsequent wafer 12 is shifted to the initial position for the alignment and focus measurements. Thus, the exposure operation illustrated in FIGS. 11A to 11F is repeated. Accordingly, the liquid-immersion exposure operation is performed on each wafer 12 in the manner described above.
FIGS. 12A, 12B, and 13 illustrate detailed examples of the wafer chuck 5C shown in FIGS. 9 and 10. Specifically, FIGS. 12A, 12B, and 13 illustrate how immersion liquid 16 may leak into a space between the undersurface of the wafer 12 and a wafer suction unit.
In the example shown in FIGS. 12A and 12B, a vacuum-chuck component 5D provided in the wafer chuck 5C vacuums the wafer 12 so as to support the wafer 12. A circumferential projection (a ring-shaped projection) 5P is provided around the outer periphery of the vacuum-chuck component 5D. If foreign matter attaches to the circumferential projection 5P or if a scratch is present on the circumferential projection 5P, a leakage 105F may occur. Such a leakage 105F may cause the immersion liquid 16 to enter the openings provided in the vacuum-chuck component 5D, and may thus lead to vacuum errors. Moreover, such a leakage 105F may, for example, lower the liquid level of the layer of the immersion liquid 16.
On the other hand, in the example shown in FIG. 13, an electrostatic-chuck component 5E provided in the wafer chuck 5C supports the wafer 12 with an electrostatic suction force. Such a technique is also problematic in that the immersion liquid 16 may enter the electrostatic-chuck component 5E so as to cause, for example, a leakage of voltage or lowering of the liquid level of the layer of the immersion liquid 16.